The ultimate goal of this project is to provide insight into the functioning of cellular machines composed of RNA and protein subunits. As a model system, we are studying the introns of the yeast mitochondrial cytochrome b pre-mRNA and the proteins that assist in their splicing. In these simple systems, the RNA components contain the active site for the splicing reaction while the protein components enhance the rate of splicing by holding the RNA in its active conformation. As a first step toward understanding the functioning of this system, we have grown crystals of a protein involved in the splicing of this pre-mRNA. These crystals diffract to 2.2 angstrom resolution, and we have determined the structure by a combination of multiwavelength anomalous scattering and multiple isomorphous replacement with anomalous scattering. Analysis of this structure is ongoing.